Typical MRAM structures have a nonmagnetic layer sandwiched between two ferromagnetic films. The two ferromagnetic films are also known as magnetic thin films. The MRAM employs the magneto resistive properties of this structure to store data. In each storage element, an MRAM employs two lines, commonly termed a word line and a sense string, in order to detect the magnetization direction of these magnetic thin films. Each string comprises a magnetic thin film that serves as a memory element, and the word line generally addresses multiple sense strings. Magnetic thin films that have a parallel moment have a low resistance and are typically assigned the ‘1’ state. Magnetic thin films having an anti-parallel moment have a high resistance and are typically assigned the ‘0’ state, but may also be assigned to the ‘1’ state.
During a read operation, a word current passes through the word line causing the magnetic layers in the sense string to rotate, thereby changing the resistance in the sense string. A sense current passes through the sense string. A sense line receives the signal from the sense string. A differential amplifier compares the signal from the sense line to a reference line to determine whether a one resistance or a zero resistance is stored in the MRAM. A differential amplifier notes the change in voltage across the sense line to determine resistive state of a storage element.
Magnetoresistive random access memory integrated circuits (MRAM IC's) have sensitive analog amplifiers that require current sources that operate within a small operation range around a selected operating point. Unfortunately, a current source operation range can vary as, for example, due to widely fluctuating manufacturing characteristics of MRAM IC's. This leads to the need for a stable reference over changes in voltage, temperature and fabrication process to control the operation point.
A useful stable reference must be stable over all the operating ranges that the device will experience. MRAM devices are typically subjected to environments that exhibit temperature, voltage, and process variations. If the analog circuitry is not stable over these conditions, it could lead to undesired or unstable operation levels at the final circuits such as the word sources and sense sources.
Some attempted approaches to provide a stable reference involve using a reference voltage that is routed throughout the entire chip. In such cases, there may be a separate reference voltage for read operations and another reference voltage for write operations.
In contrast to other approaches, the present invention employs a current reference that is stable over the operating range of the MRAM device to control operating levels in selected circuitry.